Make solidus_core depend on actionmailer and activerecord instead of rails

[BenMorganIO]

We originally closed #2239 because we thought we were going to get it in from #2236 but never did. So... Here's a new PR!

---

Before:

$ bundle
Bundle complete! 30 Gemfile dependencies, 126 gems now installed.

After:

$ bundle
Bundle complete! 30 Gemfile dependencies, 123 gems now installed.

[jhawthorn]

Seems good to me 👍

[cbrunsdon]

Rails is still being used and called directly in many places in core, (for instance:

solidus/core/lib/spree/preferences/store.rb

Line 9 in 27c0375

@cache = Rails.cache

).

Just removing the gems doesn't remove our dependence on it, we're just counting on other places to include it for us, which isn't a good practice IMO.

[BenMorganIO]

@cbrunsdon nope... Wait, isn't that from railties?

Rails only includes the README.md.

https://github.com/rails/rails/blob/master/rails.gemspec#L21

And for the Rails.cache method:

https://github.com/rails/rails/blob/master/railties/lib/rails.rb#L38

[cbrunsdon]

At the very least I think we'll need to add activesupport, activejob, and railties, which are the three I know we depend on inside of core.

[cbrunsdon]

@BenMorganIO aye, Rails is from railties, but we don't have that dependency, we get it through other gems that do explicitely have it listed, which I don't think is good to rely on.

[jhawthorn]

At the very least I think we'll need to add activesupport, activejob, and railties, which are the three I know we depend on inside of core.

We do get these from other gems, but it would make sense to add them to our deps

[cbrunsdon]

railties too please

Feedback addressed

[cbrunsdon]

I think that is a comprehensive list now of everything we need 👍